feat: Add exact phrase search support to FTS

[jdvmi00]

Adds support for quoted phrases in FTS search queries, enabling exact phrase matching alongside the existing prefix matching behavior.

Syntax

Query	Behavior
hello world	Prefix match: "hello"* AND "world"* (existing)
"hello world"	Exact phrase: "hello world" (new)
meeting "Q1 planning"	Mixed: "meeting"* AND "q1 planning" (new)

Changes

• src/store.ts: Updated buildFTS5Query() to parse quoted phrases and pass them to FTS5 as exact matches (no wildcard), while unquoted terms continue as prefix matches
• src/store.ts: Added sanitizeFTS5Phrase() helper that preserves spaces within phrases while removing FTS5 special characters
• src/store.ts: Exported buildFTS5Query for testability
• src/store.test.ts: Added unit tests for buildFTS5Query covering quoted phrases, mixed queries, unicode, case normalization, and edge cases
• src/store.test.ts: Added integration tests for searchFTS with exact phrases

Examples

# Find exact phrase
qmd search '"project timeline"'

# Mix exact phrase with regular terms
qmd search 'meeting "Q1 planning"'

# Multiple exact phrases
qmd search '"data migration" "caboodle express"'

Notes

• Phrases are lowercased to match FTS5 tokenization
• Special characters within phrases are sanitized to spaces
• Empty quoted phrases are ignored
• Fully backward compatible — unquoted queries work exactly as before