Bump version to 2.7.8

[KodeStar]

Bumps config/app.php version from 2.7.7 to 2.7.8 for the next release, which collects the fixes in this batch:

• Include tags in item export and restore them on import #1567 — include tags in item export / restore on import (Export to include 'tag' #1555)
• Harden against host header injection and open redirect (CVE-2025-50578) #1568 — host header injection & open redirect hardening (CVE-2025-50578: Host Header Injection & Open Redirect in linuxserver/heimdall:latest #1451, CVE-2025-50578)
• Update security-flagged dependencies and require PHP 8.4 (#1564) #1569 — security dependency updates + PHP 8.4 baseline (Critical CVEs #1564)
• Return graceful output from get_stats instead of a 500 #1570 — graceful get_stats output instead of 500 (Komga app stats not working (error 500) #1558)
• Fix CI: pin Node 24 and install frontend deps with npm ci #1571 — CI fix (Node 24 + npm ci) — already merged
• Add a configurable default tag for the dashboard #1572 — configurable default dashboard tag ([Enhancement] Select starting tag group when Treat Tags As: Tags is selected #1556)

The in-app version display reads config('app.version'), so no other change is required.

[KodeStar]

Closing this — 2.7.6 is the currently released version, so 2.7.8 doesn't line up, and the next bump will be 2.8.0 alongside the planned Laravel 13 upgrade. Leaving the branch in place for now.