fix: scope an approval bypass to the tool, not to its whole connector

Approving one tool call with "15 min" or "Session" registered a bypass whose
scope was *inferred* from the call's metadata: a registered category if it had
one, otherwise its MCP server. For a connector tool that meant the whole
connector — so approving `mcp__gmail__modify_message` (labelling, archiving:
what an assistant tidying a mailbox does constantly) silently un-gated
`mcp__gmail__send_message` for the rest of the conversation, straight through
the explicit `require` rule written for it. An email went out with no prompt;
the only trace was an INFO line, since bypasses live in RAM.

A human answering a card has read one call. That call is the widest thing the
click may authorise, so the scope is now always the tool itself and is never
guessed. The wider scopes stay in the enum and stay reachable through the REST
`bypass_scope` field, where naming one is deliberate.

Both fallbacks now narrow instead of widening: a scope that cannot be honoured
(a category-less tool, a non-MCP one) and an unknown scope string both degrade
to the tool, where they used to fall through to a session-wide bypass. Only a
literal "all" disables the gate session-wide.

The buttons said "skip similar requests" without ever defining "similar"; they
now name the tool.

web/i18n/en.js

@@ -321,8 +321,8 @@ export default {
   'approval.reject':    'Deny',
   'approval.confirm_reject': 'Confirm deny',
   'approval.reject_hint':   'Optional: say why (the assistant will read it)',
-  'approval.bypass_15':  'Allow and skip similar requests for 15 minutes',
-  'approval.bypass_all': 'Allow and skip all requests for this session',
+  'approval.bypass_15':  'Allow, and stop asking for this same tool for 15 minutes',
+  'approval.bypass_all': 'Allow, and stop asking for this same tool for the rest of this conversation',
 
   // ── Login ──────────────────────────────────────────────────────────────────
   'login.title':    'Welcome back',