fix(ui): restore workspace shortcuts and refine model settings

[mrsibe]

What does this PR do?

Restore workspace panel shortcuts, keep citation highlights aligned when resizing the reader, and separate chat and embedding model settings.

Why?

Panel shortcuts should expand/collapse panels just like their header buttons, while opening a citation should only reveal the Library. Closing a notebook should not compete with Escape dismissal or the window-close shortcut. Reader resizing should not leave citation highlights at stale positions, and each model capability should have a dedicated settings page.

What changed?

• Restore Library/Notes panel toggles (Ctrl/Cmd+[ and Ctrl/Cmd+]) and remembered panel widths; citation navigation only expands the Library.
• Change the default close-notebook shortcut to Ctrl/Cmd+D. Migrate the retired Escape and CommandOrControl+W bindings while preserving enabled state and other custom bindings.
• Attach and clean up keyboard listeners immediately, and handle the new-notebook shortcut on the home page.
• Remeasure text citation highlights on reader resize and align PDF/text reader scrollbar styling.
• Enlarge the home greeting and split model settings into chat and embedding pages. Remove the prompt-editing entry while retaining the underlying configuration.
• Update DESIGN.md and both shortcut locales; add shortcut migration regression tests.

How was this tested?

Re-run on the current PR branch:

• npm run build — passed, including the required npm run typecheck.
• npm test — passed: 484 tests, no failures or skips.
• git diff --check main...HEAD — passed.

Not yet verified: physical-keyboard workflows, visual checks, npm run build:unpack, and npm run smoke:packaged. Packaged-app checks are required because this PR changes Electron main-process shortcut handling. Automated checks do not establish that these runtime workflows work.

Screenshots / recordings

Pending: before/after screenshots or recordings for panel toggling, reader resizing/citation highlighting, the home greeting, and the split model settings. These meaningful UI changes still need the visual evidence required by CONTRIBUTING.md.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow.
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable
• npm run build:unpack passes.
• npm run smoke:packaged passes.