Redesign assistant editor and file-connector UX

[philmerrell]

Summary

Restyles the assistant editor and the file-source browser modal to the rounded-2xl list/form design language used by the redesigned admin pages (manage-models, tools), and reworks how documents are added to an assistant.

Editor restyle

• Form column restyled to convention tokens: rounded-2xl inputs/buttons, text-sm/6 body text, blue accent, flat <section> blocks divided by border-t (no boxed cards).
• Uploaded documents rendered as a single divide-y bordered list instead of separate cards.
• Form column given a bg-gray-50 / dark:bg-gray-900 surface so the white inputs read as raised.

File-connector UX

• Available file-source connectors are surfaced as buttons above the drop zone instead of being buried behind a generic "Import from a connector" button that opened a modal source-list.
• Clicking a connector opens the browser dialog targeted at that connector — straight into its folder browser (or an inline Connect prompt), skipping the in-modal source picker. Opening the dialog without a connector still works as before.
• The full drop zone collapses to a compact "Add files" control once a document exists or is uploading; both device and connector uploads remain available.

Modal restyle

• File-source browser modal restyled to the same conventions: rounded-2xl panel, divide-y bordered lists, convention button variants.

Skill doc

• New tailwind-ui skill reference references/app-conventions.md documenting the list/form design tokens.

Test plan

• /assistants/new — connector buttons appear above the drop zone (none if the file-source feature isn't configured).
• Click a connected connector → modal opens straight into its folders, no source-picker step, no "All sources" link.
• Click a not-connected connector → modal opens with the inline Connect prompt.
• Upload or import a file → drop zone collapses to a compact "Add files" control; can still add more via device and connector.
• Edit an assistant with existing documents → compact "Add files" shown from the start.
• Verify rounded-2xl styling, list dividers, and button styles on the editor and modal in both light and dark mode.

Notes

• Verified with ng build (development) — compiles clean. The assistants component specs were not run: they fail on unmodified develop with a pre-existing TestBed compileTestModule error. assistant-form.page.spec.ts gained a FileSourceService mock so it will pass once that environment issue is resolved.

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