feat: configurable mouse gestures with dedicated settings page

[knipknap]

Closes #241 (provides the workaround; see note below)

Summary

Mouse navigation gestures are now configurable in a dedicated Mouse Gestures page in the settings dialog. The architecture is context-based so it covers the 2D canvas, the 3D canvas, the sketcher addon, and gestures contributed by future addons.

Architecture

New package rayforge/ui_gtk/gestures/:

• GestureSpec / GestureSlot / GestureContext (model.py) — a declarative model of rebindable gestures. A spec identifies a physical input (click/drag/scroll + button + modifier mask, serialized e.g. as "drag+shift+middle"); a slot is a rebindable meaning ("Pan the view") with a default binding; a context groups slots per interaction area (canvas2d, canvas3d, sketcher).
• gesture_registry (registry.py) — an addon-capable registry following the existing SettingsPageRegistry/ContextMenuExtensionRegistry pattern, registered in REGISTRY_TABLE so addon contributions are cleaned up automatically on unload. Resolves the effective binding per slot from Config.gesture_bindings with default fallback, and detects binding conflicts within a context.
• GestureRouter (router.py) — attaches one any-button drag gesture, one any-button click gesture, and a scroll controller to a widget. On each input it resolves the actual (button, modifiers) against the context's bindings: a match claims the sequence and dispatches to the slot's handlers; no match denies the sequence, leaving existing canvas behavior (element selection, tools, etc.) untouched.

Wiring

• WorldSurface routes pan/zoom/context-menu/reset-view through a router keyed by a context_id class attribute; WorkSurface uses canvas2d, SketchCanvas overrides it to sketcher.
• CameraController routes orbit/pan/Z-rotate/zoom through the router. The previous Shift-modifier distinction became two explicit slots (orbit = middle-drag, pan = Shift+middle-drag), each independently rebindable.
• The sketcher addon registers the sketcher context (pan, zoom, tool menu, reset view) via its existing registration path — no new hookspec needed.
• Space+drag panning on the 2D canvas remains hard-coded for now (it is intertwined with element-drag handling).

Settings page

GesturePreferencesPage (built into SettingsWindow, position after General) renders one group per registered context and rebuilds live when the registry changes, so addon-contributed gesture contexts appear/disappear automatically. Each slot row opens a capture popover ("press a mouse button now, or scroll"), with conflict rejection, per-slot unassign/reset-to-default, and row labels that update live.

Persistence

Config.gesture_bindings (context_id -> slot_id -> spec string | None for unassigned), auto-saved through the existing config mechanism; malformed or unknown entries are dropped on load and unknown slots are legal (e.g. written by a disabled addon).

Notes

• Pan not working. #241: the reported pan breakage itself needs its own root-cause fix, but this feature gives affected users a workaround by remapping pan to a button/modifier combination that their hardware/driver delivers.
• Editing/selection gestures (left-click select, drag-move, sketch tools) intentionally remain hard-coded; the model leaves room to add them as slots later.
• Docs updated in website/docs/ui/settings.md.

Test plan

• New unit/UI tests: gesture spec serialization + validation, registry resolution/conflict/cleanup, router dispatch (claim/deny, modifier discrimination, unassigned slots), preferences page rebuild and live label updates, config round-trip + malformed-entry handling (tests/ui_gtk/gestures/, tests/core/test_config.py).
• Updated existing CameraController tests to the new per-slot handler API (33/33 pass).
• Full suites: tests (4867 passed), tests/ui_gtk -m ui under xvfb (627 passed), builtin addon tests (same result as main).
• Lint: ruff check, flake8 clean; translation compile step verified.