chore: release v0.4.0

[xsmyile]

Release v0.4.0 — version bump (workspace 0.3.2 → 0.4.0).

Highlights since v0.3.2:

• feat(ui): reduce friction in inbox and message preview (feat(ui): reduce friction in inbox and message preview #36)
• feat(ui): filterable tag dropdown (appears past 8 tags) (fix(ui): smooth theme switch + filterable tag dropdown #42)
• fix(ui): smooth theme switch — no toolbar flash (fix(ui): smooth theme switch + filterable tag dropdown #42)
• fix: SMTP and API hardening — slowloris/panic guards, stricter CORS, created_at index (fix: SMTP and API hardening #37)
• fix(deps): clear cargo/npm audit advisories; drop unmaintained rustls-pemfile (fix(deps): clear dependency audit advisories #41)
• docs: accuracy fixes across the site (docs: remove prose em dashes, fix STARTTLS/armv7 accuracy, rewrite ui/README #38, docs: hide non-functional "Try it out" button from API reference #39, docs: fix stale AGENTS.md commands and cross-refs #40)

Tagging v0.4.0 after merge triggers the release pipeline (binaries, Docker, GitHub release, Homebrew tap).