feat: sidebar instance filter with / shortcut

[Teamingzooper]

Summary

Adds a text input at the top of the sidebar that filters visible instances by substring match against the instance name. Particularly useful for users with 8+ instances where scanning the sidebar gets slow.

Branch base: `feat/instance-hibernate` — sixth in the stack. Merge order: #6 → #7 → #8 → #9 → #10 → this.

UX

• `/` key focuses the filter from anywhere in the app (skipped when typing in another input, textarea, or contenteditable so it doesn't steal slashes mid-sentence).
• `Esc` in the filter clears the text; if already empty, blurs the input.
• `Enter` activates the first matching instance and blurs the input.
• `×` clear button appears when there's text.
• Filter is hidden in compact-sidebar mode (sidebar is too narrow to render it usefully).

Filter semantics

• Match is substring, case-insensitive, on `instance.name`.
• While a filter is active, groups with no matching instances are hidden — so the user sees only the matches.
• When the filter is empty, all groups stay visible — including empty ones, so the "Drop an instance here" affordance for new groups still works.

Implementation

Purely renderer-side. Local React state in `Sidebar.tsx`, no store changes, no IPC, no main-process changes, no schema additions.

Test plan

• `npm run typecheck` — clean
• `npm run test` — 178/178 passing
• `npm run build` — renderer + main both clean
• Manual smoke: with several instances open, press `/` → filter focuses. Type a few characters → instance list shrinks to matches. Hit Enter → first match activates. Hit `/` again → refocuses with current text selected. Type "xx" (no matches) → all groups hide, no instances show.
• Manual smoke: try `/` while typing in a rename input or in the embedded Gmail webview — it should NOT steal focus.

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