Flagged marker for history entries

Last of the "should build soon" items from the Burp/ZAP/Caido gap
research - Burp's row highlighting and Caido's Findings both serve
the same real workflow: mark something interesting mid-engagement,
revisit later. Scoped to a boolean flag (★) rather than full free-text
notes/comments, which would need their own text-input overlay for
comparatively modest extra value over a simple marker - tracked as a
real follow-up in PLAN.md, not dropped silently.

internal/store: history gains a flagged column (migrated in for
existing databases the same way source was) plus Store.SetFlagged and
Summary/Entry.Flagged. Search's structured-filter layer (added last
commit for status:/source:) gains flagged:true/false alongside them -
extractStructured already existed for exactly this kind of "pull it out
before it reaches FTS5" filter. internal/ipc gains a "set_flagged"
request. cmd/mitmux: 'f' toggles the flag on the selected history row
(applied optimistically to local state, persisted async - a drift
between local and server state on failure is an acceptable trade-off
for a marker this low-stakes), shown as a ★ column in the list and in
the detail view's title.

store_test.go covers the flagged: parsing (true/false spellings, and
a "looks like it but isn't" case - flagged:maybe - falling through as
literal search text, matching the existing pattern for status:).

Verified live: toggling 'f' shows the star immediately, flagged:true
correctly filtered to just that entry, and a direct SQLite check
confirmed the flag actually persisted to the database (flagged=1),
not just reflected in local UI state.

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