fix(web): restore sidebar access in zoomed empty state

[Pearcekieser]

Summary

• Allow the thread sidebar resize guard to accept shrinking even when the stored sidebar width is wider than the viewport/main content area.
• Show the no-active-thread sidebar trigger whenever the sidebar is currently hidden, including Electron/mobile-sheet mode after zooming in.
• Add focused tests for the sidebar width predicate and no-active-thread trigger visibility predicate.

Why

When the app is zoomed in enough to enter the mobile sidebar layout and no thread is selected, the sidebar becomes a closed sheet. The Electron no-active-thread header did not render SidebarTrigger, leaving only "Pick a thread to continue" with no visible way to reopen the sidebar and select a thread.

The existing width guard also rejected every drag when the sidebar width exceeded the wrapper width, so users could not shrink back out of a bad persisted/resized sidebar state.

Checks

• bun fmt
• bun lint (passes with existing warnings)
• bun typecheck
• bun run test -- src/components/AppSidebarLayout.test.ts src/components/NoActiveThreadState.test.ts

Test Notes

• bun run test currently fails outside this change in scripts/mock-update-server.test.ts with a timeout in serves files from the configured root.
• bun run test --filter=@t3tools/web currently fails outside this change in src/environments/runtime/service.addSavedEnvironment.test.ts with a timeout in rolls back persisted metadata when bearer token persistence fails.

Note

Restore sidebar access in the 'No active thread' empty state when sidebar is collapsed

• The NoActiveThreadState component now conditionally renders SidebarTrigger only when needed: on desktop when the sidebar is collapsed, and on mobile when the sheet is closed. Previously the trigger was always shown on non-Electron and never shown on Electron.
• A shouldShowNoActiveThreadSidebarTrigger helper encapsulates this visibility logic, with tests added in NoActiveThreadState.test.ts.
• shouldAcceptThreadSidebarWidth in AppSidebarLayout.tsx is updated to allow all shrink operations unconditionally, while still enforcing the minimum main content width on expansion.

^{Macroscope summarized caf8c4f.}

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[Pearcekieser]

Superseded by #2884, which uses the date-prefixed worktree/branch requested for this fix.