Add a named canvas with click-to-edit toolbar field

[mucow24]

Summary

• Adds a name field to ProjectDocument (default "Untitled canvas") with a new set_canvas_name reducer command — undoable like set_background.
• Renders the name prominently in the top toolbar between the File menu and the Canvas focus icon. Clicking the name opens an inline input that commits on Enter or blur, cancels on Escape, and falls back to the default if the user clears it.
• Drives PNG export and JSON save filenames via a shared sanitizeBasename helper, so multiple projects no longer collide on `billboard-export.png` / `billboard-project.json`.

Notes for reviewers

• Schema treats `name` as optional and the normaliser fills the default; legacy saved files load without changes.
• File IO primitives stay dumb — sanitisation lives in src/editor/io/filename.ts with focused unit tests. `downloadCanvasAsPng` already took a `fileName`; `downloadProject` gains an optional one with the previous default preserved.
• App.tsx dispatches `set_canvas_name` inline, matching how `set_background` is wired (no new store action).
• Two pre-existing assertions on `downloadCanvasAsPng` (App integration + useEditorController) updated to include the new filename argument.

Test plan

• `npm run lint && npm run typecheck` — clean
• `npm test` — 657 / 657 pass (new tests for sanitiser, reducer, schema/normaliser/codec/defaults, toolbar, project file)
• `npm run e2e -- editor.files-and-persistence` — new spec exercises default → rename → save filename → reload restoration → load file restores name → export filename
• Manual preview verification (per project convention) — confirmed by author

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