v0.12.4 — 15 bug fixes (CRITICAL/HIGH/MEDIUM)

CRITICAL:
- C1: SetAppBackupBulk data loss + nil map panic (settings.go)
- C2: UpdateStackConfig nil Env map panic (deploy.go)
- C3: ValidateDump missing scanner.Err() check (dbdump.go)

HIGH:
- H1: nextDailyRun DST bug — use time.Date(day+1) not Add(24h)
- H2: Cache Europe/Budapest timezone with sync.Once in scheduler
- H3: settings.save() leaks .tmp file on WriteFile failure
- H4: SetNotificationPrefs nil pointer panic
- H5: appDirSize + getDirSizeBytes ignore Sscanf return value
- H6: getDirSizeBytes has no timeout — add 30s context
- H7: dbdump.go tmpFile not using defer Close
- H8: UpdateCrossDriveStatus misleading comment

MEDIUM:
- M1: Replace custom containsBytes with strings.Contains
- M2: scheduler.Every() validates interval > 0
- M3: executeJob panic recovery now sets LastRun
- M4: logPostStartStatus copies env slice before goroutine
- M5: Cache timezone in web package via getTimezone() sync.Once

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,32 @@
 ## Changelog
 
+### What was just completed (2026-02-18 session 40)
+- **v0.12.4 — Correctness & Robustness Bug Fixes (TASK.md — 15 bugs fixed):**
+
+  **CRITICAL fixes (data loss, panics):**
+  - **C1: `SetAppBackupBulk` data loss + nil map panic** — Fixed: now updates map IN PLACE instead of replacing it, so stacks absent from the input are preserved. Added nil guard for `s.AppBackup`. (`internal/settings/settings.go`)
+  - **C2: `UpdateStackConfig` nil Env map panic** — Added nil check `if appCfg.Env == nil { appCfg.Env = make(...) }` before the field assignment loop. (`internal/stacks/deploy.go`)
+  - **C3: `ValidateDump` missing scanner.Err() check** — Added `if err := scanner.Err()` check after the scan loop so I/O errors don't silently mark a partial dump as valid. (`internal/backup/dbdump.go`)
+
+  **HIGH fixes (logic errors, resource leaks):**
+  - **H1: `nextDailyRun` DST bug** — Replaced `next.Add(24 * time.Hour)` with `time.Date(day+1, ...)` for correct scheduling across Europe/Budapest DST transitions. (`internal/scheduler/scheduler.go`)
+  - **H2: `nextDailyRun` repeated `LoadLocation`** — Cached timezone in package-level `sync.Once` variable; `getBudapestLocation()` now loaded only once. (`internal/scheduler/scheduler.go`)
+  - **H3: `settings.save()` .tmp file leak** — Added `os.Remove(tmpPath)` cleanup on `WriteFile` failure path. (`internal/settings/settings.go`)
+  - **H4: `SetNotificationPrefs` nil pointer panic** — Added nil guard at start of function, returns error instead of panicking. (`internal/settings/settings.go`)
+  - **H5: `appDirSize` ignores `Sscanf` return value** — Now checks `n != 1` and returns `(0, "?")` on parse failure. Same fix applied to `getDirSizeBytes` in `stacks/delete.go`. (`internal/backup/appdata.go`, `internal/stacks/delete.go`)
+  - **H6: `getDirSizeBytes` no timeout** — Added `exec.CommandContext` with 30s timeout. Added `"context"` import. (`internal/stacks/delete.go`)
+  - **H7: `dbdump.go` tmpFile not using `defer Close`** — Replaced explicit `tmpFile.Close()` call with `defer tmpFile.Close()` so the file handle is released even on panic. (`internal/backup/dbdump.go`)
+  - **H8: `UpdateCrossDriveStatus` misleading comment** — Updated comment to accurately describe the "does nothing if nil" behavior instead of claiming it "creates one if nil". (`internal/settings/settings.go`)
+
+  **MEDIUM fixes (code quality, edge cases):**
+  - **M1: Custom `contains`/`containsBytes` replaced** — Removed bespoke `containsBytes` and simplified `contains` to delegate to `strings.Contains`. Added `"strings"` import. (`internal/notify/notifier.go`)
+  - **M2: `scheduler.Every()` doesn't validate interval** — Added early return with error log if `interval <= 0` to prevent panic in `time.NewTicker`. (`internal/scheduler/scheduler.go`)
+  - **M3: `executeJob` panic recovery missing `LastRun`** — Panic recovery defer now also sets `job.LastRun = time.Now()` so the job status shows a timestamp after a panic. (`internal/scheduler/scheduler.go`)
+  - **M4: `logPostStartStatus` goroutine captures env by reference** — Copies the env slice before launching the goroutine (`envCopy`). (`internal/stacks/manager.go`)
+  - **M5: Multiple `time.LoadLocation` calls in web package** — Added package-level `getTimezone()` with `sync.Once` in `funcmap.go`. Replaced all `time.LoadLocation("Europe/Budapest")` calls in the web package with `getTimezone()`. (`internal/web/funcmap.go`, `internal/web/handlers.go`)
+
+  **Files modified (8):** `internal/settings/settings.go`, `internal/stacks/deploy.go`, `internal/backup/dbdump.go`, `internal/scheduler/scheduler.go`, `internal/backup/appdata.go`, `internal/stacks/delete.go`, `internal/stacks/manager.go`, `internal/notify/notifier.go`, `internal/web/funcmap.go`, `internal/web/handlers.go`
+
 ### What was just completed (2026-02-17 session 39)
 - **v0.12.3 — Security & Correctness Bug Fixes (TASK.md — 33 bugs fixed):**