First pass at color heirarchy and some light UI polish

[jzimdars]

[Copilot]

Pull request overview

Updates TUI color hierarchy, mail navigation, collection browsing, message-list presentation, and rich HTML rendering.

Changes:

• Introduces theme-aware chrome, muted text, active-tab, and alert styling.
• Adds numeric mailbox shortcuts, a Collections picker, and Imbox seen/unseen sections.
• Renders pasted HTML Trix attachments as text and expands test coverage.

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Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/tui/tui.go	Refreshes help after navigation changes.
internal/tui/tui_test.go	Tests styling, layout, shortcuts, and help.
internal/tui/styles.go	Defines the revised color hierarchy.
internal/tui/search.go	Applies muted search-label styling.
internal/tui/nav.go	Revises shortcuts, tabs, rules, and header layout.
internal/tui/mail.go	Adds Collections navigation and Imbox grouping.
internal/tui/mail_test.go	Tests Collections and mailbox behavior.
internal/tui/loading.go	Applies muted loading-label styling.
internal/tui/folders.go	Applies muted form-label styling.
internal/tui/content.go	Groups and restyles posting rows.
internal/tui/contacts_list.go	Updates contact-list secondary styling.
internal/tui/contact_form.go	Updates contact-form secondary styling.
internal/tui/compose.go	Updates compose-form secondary styling.
internal/tui/collections.go	Implements the Collections picker.
internal/tui/collections_test.go	Tests modal width constraints.
internal/tui/calendar_views.go	Updates calendar muted styling.
internal/tui/bulk_reply.go	Updates bulk-reply secondary styling.
internal/htmlutil/htmlutil.go	Renders inline HTML Trix attachments.
internal/htmlutil/htmlutil_test.go	Tests rich HTML and file attachments.

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