feat: #tag support for notes

[oobagi]

Problem

There's no way to categorize or discover notes by topic. Users have to remember which notebook a note is in or search by title.

Goal

Add Obsidian-style #tag support — inline tags in note content that are parsed, rendered as clickable pills, and searchable.

Syntax

Some text with #project-alpha and #urgent tags inline.

Behavior

• Edit mode: Tags are typed as plain text (#tag-name). Rendered with subtle highlighting so they're visually distinct while editing.
• View mode: Tags render as styled clickable pills. Clicking a tag opens a filtered list of all notes containing that tag.
• Search: Tags are discoverable via the existing search feature — searching #tag-name filters to notes containing that tag.

Tasks

• Parse #tag-name as an inline element (alphanumeric + hyphens, no spaces)
• Serialize tags back to #tag-name syntax
• Round-trip tests
• Edit mode: subtle syntax highlighting for tags
• View mode: render tags as styled pills
• View mode: clicking a tag filters the note list to notes containing that tag
• Search integration: #tag query matches tag content
• Add TagStyle to theme system (pill color, hover state)

Design considerations

• Tags live in content, not metadata — no separate tag database needed (like Obsidian)
• Tag names: lowercase, alphanumeric + hyphens, case-insensitive matching
• Tag filtering view: reuse existing note list with a filter applied, not a separate screen
• No autocomplete for v1 — can add later

Scope

Type: enhancement
Size: medium