feat(browser): mark finished items with a check cap, not a thick bar

[ajslater]

Refs #856.

Alfons' feedback on the v2.4.0 read state: the 6px grey finished bar is hard to tell from the 3px reading bar when you scan a grid of covers.

Changes

• Check cap. Every read bar is now 3px. A finished comic or all-read collection ends its bar in a 10px grey check circle, the same grey as the fill. The bar runs under the cap to its centre, so the two join with no gap. The fill still shows the real bookmark position, so a comic marked read but never opened is an empty track with its cap.
• No dimmed captions. Read titles and subtitles keep their normal colour.
• One component. ReadStateBar (frontend/src/components/read-state-bar.vue) now draws the bar for both the browser card and the metadata dialog. They had duplicated its CSS.
• Metadata dialog fix (separate commit). The dialog's read bar has been invisible since v2.4.0 because the absolutely positioned cover image painted over it. It now sits 1px under the cover, as on the card. The dialog's buttons move down about 16px.

Testing

• make lint passes. make test-frontend passes 940 tests, including the new read-state-bar.test.js.
• Checked in a real browser against a throwaway library, at desktop and phone widths:
  • The cap appears only on an all-read folder, a comic marked read but never opened, and a finished comic being re-read.
  • Reading, mixed and unread items have no cap.
  • All bars measure 3px, the bar ends at the cap's centre, and nothing below the cover moves.
• No Python changed, so pytest was not run.

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