feat(files): a Files section over the caller's whole space

Until now file browsing existed only inside a project, and the two memory
stores were reachable only by the agent's tools. `#files` is the general
surface: home, both memory stores, the shared folders and projects the
caller belongs to, plus the read-only skills and docs trees.

The root is virtual, and that is the design. Anchoring at `~` is wrong:
the explorer reads host-side, while `shared/`, `projects/`, `skills/` and
`docs/` are bind mounts inside the container — a page rooted at the home
would show less than the user has with no way to reach the rest, and on
native Linux would show Docker's empty mountpoint stubs, a door that
appears to work and leads nowhere. So level 0 is a synthetic list from the
new `GET /api/files/roots`, serialized from the caller's `UserFs` plus the
two virtual memory roots. It sends `kind`, never a label: labels are copy
and get translated.

`GET /api/files/dir` now answers `{ path, can_write, entries }`, and a
memory path is classified before `resolve_view_path` (which refuses one)
and listed from `memory_docs`: one level derived from the flat key space
by the pure `memory_docs::immediate_children`, over a single query whose
unslashed prefix also spots an exact note as "not a directory". Memory is
read-only from the page — every writer routes through `resolve_view_path`,
and `shared-memory/*` is `@fs_write require` for the agent, so a button
that walks past that rule is a decision of its own.

The explorer moves out of projects into `shared/file-explorer.js`, taking
`root` + `rootLabel` and reading `can_write` from the listing rather than
from its host: writability changes per branch and comes from the same
`UserFs::can_write_to` the server rejects writes with, so the buttons
offered and the writes accepted cannot disagree. Deep-linking needed it
steerable without a two-way binding, hence `rel` in and
`explorer-navigate` out — the event fires only for a click, never for a
`rel` the host set, so echoing it back is a no-op.

The URL carries the agent path of the open folder in one parameter, the
same vocabulary the assistant uses, so a link is shareable and pasteable
into a conversation; which root it belongs to is derived, not stored.

docs/: a new files.md, plus two pages this made false — shared-folders.md
claimed in three places that a shared folder has no explorer, and
memory.md never said a user can now read their own notes.

web/i18n/en.js

@@ -8,6 +8,7 @@ export default {
   'nav.inbox':         'Inbox',
   'nav.dashboard':     'Dashboard',
   'nav.projects':      'Projects',
+  'nav.files':         'Files',
   'nav.tasks':         'Task Manager',
   'nav.tasks.running':   'Running Tasks',
   'nav.tasks.cron':      'Cron Jobs',
@@ -266,26 +267,40 @@ export default {
   'projects.share.access.readwrite':'Read & write',
   'projects.tabs.files':              'Files',
   'projects.tabs.sharing':            'Sharing',
-  'projects.files.col.name':          'Name',
-  'projects.files.col.created':       'Created',
-  'projects.files.col.modified':      'Modified',
-  'projects.files.col.size':          'Size',
-  'projects.files.empty':             'This folder is empty.',
-  'projects.files.refresh':           'Refresh',
-  'projects.files.drop':              'Drop files here to upload',
-  'projects.files.btn.new_folder':    'New folder',
-  'projects.files.btn.upload':        'Upload',
-  'projects.files.btn.download':      'Download ZIP',
-  'projects.files.uploading':         'Uploading…',
-  'projects.files.action.download':   'Download',
-  'projects.files.action.rename':     'Rename',
-  'projects.files.action.delete':     'Delete',
-  'projects.files.confirm.delete_file': 'Delete "{name}"?',
-  'projects.files.confirm.delete_dir':  'Delete the folder "{name}" and everything inside it?',
-  'projects.files.modal.mkdir':       'New folder',
-  'projects.files.modal.rename':      'Rename "{name}"',
-  'projects.files.modal.name':        'Name',
-  'projects.files.error.name':        'Enter a valid name (no slashes).',
+
+  // ── File explorer (shared component: projects, Files section) ───────────────
+  'files.col.name':          'Name',
+  'files.col.created':       'Created',
+  'files.col.modified':      'Modified',
+  'files.col.size':          'Size',
+  'files.empty':             'This folder is empty.',
+  'files.refresh':           'Refresh',
+  'files.drop':              'Drop files here to upload',
+  'files.btn.new_folder':    'New folder',
+  'files.btn.upload':        'Upload',
+  'files.btn.download':      'Download ZIP',
+  'files.uploading':         'Uploading…',
+  'files.action.download':   'Download',
+  'files.action.rename':     'Rename',
+  'files.action.delete':     'Delete',
+  'files.confirm.delete_file': 'Delete "{name}"?',
+  'files.confirm.delete_dir':  'Delete the folder "{name}" and everything inside it?',
+  'files.modal.mkdir':       'New folder',
+  'files.modal.rename':      'Rename "{name}"',
+  'files.modal.name':        'Name',
+  'files.error.name':        'Enter a valid name (no slashes).',
+  'files.title':             'Files',
+  'files.note.roots':        'Everywhere you can go: your home, your two memory stores, and the folders shared with you. These are the same paths the assistant uses.',
+  'files.back':              'All files',
+  'files.badge.readonly':    'Read-only',
+  'files.error.unknown_path': '"{path}" is not one of your folders.',
+  'files.root.home':           'Home',
+  'files.root.user_memory':    'Personal memory',
+  'files.root.shared_memory':  'Shared memory',
+  'files.root.shared':         'Shared folder',
+  'files.root.project':        'Project',
+  'files.root.skills':         'Skills',
+  'files.root.docs':           'Documentation',
 
   // ── Project detail ──────────────────────────────────────────────────────────
   'project_board.back':               'Projects',
@@ -1226,6 +1241,7 @@ export default {
   'common.cancel':  'Cancel',
   'common.loading': 'Loading…',
   'common.saved':   'Saved',
+  'common.create':  'Create',
 
   // ── Shared Folders (blueprint §6) ────────────────────────────────────────────
   'nav.shared_folders':       'Shared Folders',