fix(security): resolve all HIGH/MEDIUM/LOW audit findings
Rust fixes (HIGH):
- symbols.rs: path validation (reject near-root, 50K file limit, symlink filter)
- memory.rs: FTS5 query quoting (prevent operator injection), 1000 fragment cap, content length limit, transaction wrapping
- budget.rs: atomic check-and-reserve via transaction, input validation, index on budget_log
- export.rs: safe UTF-8 truncation via chars().take()
- git_context.rs: reject paths starting with '-' (flag injection)
- branch_policy.rs: action validation (block|warn only), path validation
Rust fixes (MEDIUM):
- export.rs: named column access (positional→named)
- budget.rs: named column access, negative value guards
Svelte fixes:
- AccountSwitcher: 2-step confirmation before account switch
- ProjectMemory: expand/collapse content, 2-step delete confirm, tags split fix
- CodeIntelligence: min 2-char symbol query, CodeSymbol rename, aria-labels
- BudgetManager: 10M upper bound, aria-label on input, named constants
- SessionExporter: timeout cleanup on destroy, aria-live feedback
- AnalyticsDashboard: SVG aria-label, removed unused import, named constant
738574b9f0