Word-count goals with progress in the status bar

[tosinamuda]

User story
As a student writing against a deadline, I want a word-count goal per document with visible progress, so that I can pace my writing sessions.

Acceptance criteria

• Set/clear a goal per document (stored in frontmatter, e.g. goal: 2000 — plain-file portable, no hidden database)
• Status bar shows words / goal with a subtle progress treatment when a goal exists; plain word count otherwise (today's behavior)
• Selecting text shows the selection's word count
• Counting matches the existing word-count pipeline (no second counter to drift)

Sub-tasks

• Frontmatter goal field read/write via the properties panel
• Status bar progress UI
• Selection count wiring
• Tests (goal parse, progress states, selection count)