Search: Use one shared field across pages

[whereareiam]

Search fields were written and styled separately on almost every page that has one, alongside two shared implementations (theme-search-field and theme-search-control) that declared the same 32px box. This moves all of them onto components/common/search/search.

Changes

• The component takes Kind: submit (default, trailing search button), icon (leading search icon) or plain. Local marks a client-side filter that is never submitted, ID and Name set the input's identity, Label overrides the accessible name, and Class adds a page layout hook.
• Migrated: profile and organization repository search, profile projects and packages, releases, tags, the Actions run filter, the Actions log search, and both diff filters.
• The rules for Forgejo's native query/mode/submit group (theme-search-control, used by repository code search) move from the theme engine into the component stylesheet, so one file owns search fields.
• Page stylesheets keep only layout for these fields (width, flex). About 100 lines of per-page field styling are removed.
• A test fails when any template other than the component writes a search input. CONTRIBUTING.md documents the variants.

Not migrated: the "Go to file" picker, which is a combobox with a shortcut hint and clear button rather than a search field.

Visual result

Intended to be unchanged. Measured in the dev preview at 1440px before and after, and against github.com:

Field	Before	After	GitHub
Box (all fields)	32px, 1px border, 6px radius	same	same
Text	14px/20px	same	same
Plain: text offset	13px	13px	13px
Icon: icon offset	8–9px	8px	8px
Icon: text offset	33px	33px	33px
Submit: button	32×30	32×30	n/a

Two deliberate differences: icon fields on Actions and diff move the icon 1px left to GitHub's offset, and the release and tag search icon is now decorative rather than a submit button, as on GitHub (Enter submits).

Checks

• bun commit passes (385 tests).
• Dev preview (http://localhost:3000, Forgejo 16.0.3): release search submits and keeps its value; the Actions run filter and both diff filters filter and restore their rows; focus rings render; every field fits a 390px viewport without horizontal overflow.
• Not exercised: profile packages (the fixture has no packages) and log-line filtering in the Actions log search (the fixture run has no loaded log lines; the field and its match counter were checked).

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