Theme/dark mode and visual refresh

[tanc]

What this changes

This brings together two related pieces of work on the web UI: a correctness fix for dark mode and accessibility, and a visual refresh. It is kept as two commits so they can be reviewed or merged separately if preferred.

Part 1, dark mode and accessibility:

• Dark mode now uses Bootstrap's native data-bs-theme attribute instead of a separate data-theme, so Bootstrap themes its own components from the --bs-* variables. The hand written override block in assets/css/piler.css is mostly gone.
• Fix the unreadable dark mode search results header. Bootstrap does not darken the .table-secondary variant, which left light text on a light background.
• Fix the dark on dark search placeholder, and theme alerts and the health "refresh period" strip for dark mode.
• Give the login inputs proper accessible names. The labels used for="floatingInput" and for="floatingPassword" and did not match the input ids.
• Name the logout link and the theme toggle, and remove a duplicate id="menu" wrapper in the common layout.

Part 2, visual refresh:

• The top navigation used 24px text at weight 100, and headings were oversized and thin. The navigation now uses a normal 15px, headings follow one scale, and nav items are vertically centred in the header.
• Nav links now have a rounded hover pill with padding around the icon and label.
• The brand palette is now a set of tokens with a refined indigo primary, softer neutral surfaces and a clearly separated dark header. The health strip was stock teal and is now part of the palette.
• Search toolbar buttons now have spacing, Search is green, and Options is much smaller.
• The tag input is now a normal Bootstrap input, the footer no longer clips it, and "With Selected" stays on one line.
• The resizer between the results and the preview pane now works, with a larger drag handle.
• Wide data tables scroll inside their container on small screens.

Verification

• Computed style checks in both themes: search header 16.35:1 in light and 10.87:1 in dark, search placeholder 6.78:1 in light and 7.99:1 in dark.
• The login fields report labels === ["Email address"].
• Light and dark were checked on login, search, health, users and message views, at desktop and phone widths.

Notes

• No PHP logic, configuration or schema changes.
• Tested against piler 1.4.9. The webui/ tree is identical between the piler-1.4.9 tag and master.

Screenshots before and after

Some examples of the changes:

Login before/after (dark)

Preview before/after (light)

Preview before/after (dark)

[tanc]

This is a superset of the other PR: #508
If this approach is acceptable then the other PR can be closed without merging as its included here