refactor: make configuration read-only at runtime and improve settings for Docker environments

[IllusionOfControl]

Description

Motivation (Docker Context)

Previously, saving settings in the Web UI attempted to rewrite the .env file via atomic rename (os.Rename).
In Docker container environments, this causes critical issues:

1. Mount errors (EBUSY: Device or resource busy): when bind-mounting an .env file into a container (volumes: [ ./.env:/app/.env ]), atomic file replacement via temporary file rename fails under Linux due to locked file inodes.
2. Environment precedence conflict: when configuration is injected via container environment variables (environment: in Compose or -e), OS environment variables take precedence over .env on startup. Changes saved through the Web UI to the file are effectively overridden and lost on restart.
3. Container immutability: in containerized and cloud deployments, configuration should be declarative, injected from the outside, and immutable (read-only) at runtime.

This PR makes the service configuration strictly read-only at runtime and completely eliminates .env mutation by the application.

Key Changes

1. Read-only configuration on backend:

   • Removed Save() and atomicWrite from internal/config/config.go.
   • Removed PUT /api/config endpoint and UpdateRuntimeConfig methods from internal/api/ and internal/app/.
   • Configuration is strictly read-only and accessible via GET /api/config.

2. Simplified read-only SettingsPanel in Web UI:

   • Converted SettingsPanel.vue from an editable form to a clean, compact key-value display.
   • Added a prominent informational callout block explaining that settings are loaded from environment variables or .env at startup and are immutable at runtime.
   • Organized settings into two clear, compact panels: Network & Access and Workers & Scheduling.

3. Camoufox HEADLESS configuration:

   • Previously, Camoufox WAA workers were hardcoded to headless mode (Headless: true).
   • Exposed HEADLESS environment variable (defaults to true), allowing visible browser window when set to false for debugging and troubleshooting.
   • Added status indicator in Web UI and documented in .env.example.

4. Comprehensive ROUTING_STRATEGY documentation:

   • Added clear, detailed descriptions for round-robin (balances load and distributes rate limits across accounts) and fill-first (sticky routing until full or cooling down before falling back) in .env.example, Web UI, and docs.

5. Support for CAMOUFOX_PATH in .env:

   • Added CAMOUFOX_PATH to recognized .env keys in configKeys so that paths specified in .env are exported to the process environment.

[IllusionOfControl]

In case of consideration of the proposal, I will make an update in this MR, taking into account my past MR #27