feat(tags): add an Edit tags dialog on Ctrl+T

Five hardcoded tags were the only ones reachable from the UI: archive,
delete, spam, flag and toggle_unread. For an application whose purpose is
organising mail by tag, applying any other one meant leaving for a
terminal. Item 26 of the usability backlog, raised by the user asking how
to add a tag and finding they could not.

One dialog rather than separate add and remove actions, at the user's
choice: filing something under a new tag while dropping inbox is one
thought, not two. Type tags to add or remove, comma separated, or clear a
checkbox to drop a tag already on the selection without retyping its
name.

Both fields complete against the tag list MainWindow already holds for
the query completer. Completion is a guard against typing shoppping
beside shopping, never a whitelist: inventing a tag is the entire point,
so any valid name goes through whether or not it exists yet.

Tri-state checkboxes carry the multi-thread case, and are where the risk
is. A tag on some selected threads shows partially checked, and leaving
it alone changes nothing; the opposite reading would silently tag threads
the user never looked at. A tag already on every thread and left checked
is likewise not a change and is not sent as one.

Tag names are validated before anything is applied, through a free
function so the rules are testable on their own. Empty, a leading dash
(notmuch's CLI reads it as removal, making such a tag a trap), whitespace
and control characters are refused by name and reason. Nothing is applied
until the whole set passes, since the user cannot tell which half of a
partial change landed.

TagDialog is pure UI: handed the vocabulary and the current state,
returning two lists, contacting no worker. That is what lets its fifteen
tests run without a notmuch database. Integration is a single call to the
existing tagSelected(), so undo, the optimistic model update, the
combined multi-row query and the completer refresh for a brand-new tag
all come for free.

One test assumption was wrong and the code was right: a case asserted
that QStringLiteral("null\0byte") truncates at the null and reads as
empty. It does not, so the null is caught as a control character. The
test was corrected rather than the validator.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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