Chrome extension polish: verify icons, popup footer, per-section docs link

[AimTune]

Small UX improvements for the Chrome extension once the toolbar icons land.

Tasks

• After icons land (see feat(vue3): @chativa/vue wrapper package (Vue 3) #2), build the extension and verify Chrome shows the toolbar icon at every DPI (16 / 32 / 48 / 128 px). Capture a fresh popup.png while we're there.
• Add a footer to the popup with two links:
  • The hosted sandbox: https://aimtune.github.io/chativa/
  • The docs hub: https://github.com/AimTune/chativa/tree/main/docs
• (Stretch) "Open docs for this section" button next to each collapsed section in the popup, mirroring the per-tab Docs link in the sandbox.

Notes

• Footer should be sticky-ish at the bottom of the popup so it does not move on scroll.
• Use the same purple gradient tokens already in apps/chrome-extension/src/popup.ts for visual consistency.

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Spun out of todos.md.