fix(toolbar): let draw/comment/inspect activate with no pages

[lklyne]

Summary

Follow-up to #181. That PR removed the hasPages-gated disabled on the Draw/Comment/Inspect toolbar buttons, but a second, main-side gate remained: sanitizeForPages() in src/main/runtime/tool-mode.ts silently collapsed annotation/inspect tools back to select whenever the canvas had zero pages.

The result: with no frame on the canvas, the Draw (pen/marker) button looked clickable but the activation died in main — activeTool never became 'draw', so the tool didn't select and its DrawToolPopup (gated on activeTool.kind === 'draw') never appeared. Same for Comment and Inspect.

This removes the page collapse entirely, so draw, comment, and inspect activate on an empty canvas — matching the always-enabled buttons. Only the drawing feature flag can veto a tool now.

It also closes a parallel gate: the right panel's "Add comments" button used annotateAvailable: pages.length > 0, which stayed disabled on an empty canvas even though the toolbar comment tool now activates there. That flag is now always true so both surfaces agree.

Why it's safe with zero pages

The inspect path (the only one that touches page content) is already defensive:

• syncInspectionState() iterates an empty pages list → no-op
• panel data reports available = pages.length > 0
• inspection-enabled is forced false when there are no pages
• hover-target handlers guard if (!page || …isDestroyed()) return

syncAnnotationState() (draw/comment) also just iterates the empty pages list. No tool crashes; inspect simply has nothing to target until a page exists.

Test plan

• Empty canvas (no pages): click Draw → tool activates and the brush/color popup appears in the top toolbar.
• Empty canvas: click Comment → comment tool activates.
• Empty canvas: open right panel → Comments → "Add comments" button is enabled and toggles comment mode.
• Empty canvas: click Inspect → inspect tool activates (panel shows nothing to inspect, no error).
• With pages present, all three tools behave exactly as before.
• pnpm typecheck passes.

🤖 Generated with Claude Code