Refactor web build process and update documentation

- Removed Node.js build artifacts from .gitignore and adjusted Makefile to reflect changes in web UI build process, now using server-rendered Go templates instead of React.
- Updated README to clarify the new web UI architecture and output formats, emphasizing the removal of the Node.js build step.
- Added a command to set the number of frames per render task in manager configuration, enhancing user control over rendering settings.
- Improved Gitea workflow by removing unnecessary npm install step, streamlining the CI process.

internal/database/migrations/000002_add_task_frame_end.down.sql

@@ -0,0 +1,31 @@
+-- SQLite does not support DROP COLUMN directly; recreate table without frame_end
+CREATE TABLE tasks_new (
+	id INTEGER PRIMARY KEY AUTOINCREMENT,
+	job_id INTEGER NOT NULL,
+	runner_id INTEGER,
+	frame INTEGER NOT NULL,
+	status TEXT NOT NULL DEFAULT 'pending',
+	output_path TEXT,
+	task_type TEXT NOT NULL DEFAULT 'render',
+	current_step TEXT,
+	retry_count INTEGER NOT NULL DEFAULT 0,
+	max_retries INTEGER NOT NULL DEFAULT 3,
+	runner_failure_count INTEGER NOT NULL DEFAULT 0,
+	timeout_seconds INTEGER,
+	condition TEXT,
+	created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+	started_at TIMESTAMP,
+	completed_at TIMESTAMP,
+	error_message TEXT,
+	FOREIGN KEY (job_id) REFERENCES jobs(id),
+	FOREIGN KEY (runner_id) REFERENCES runners(id)
+);
+INSERT INTO tasks_new (id, job_id, runner_id, frame, status, output_path, task_type, current_step, retry_count, max_retries, runner_failure_count, timeout_seconds, condition, created_at, started_at, completed_at, error_message)
+SELECT id, job_id, runner_id, frame, status, output_path, task_type, current_step, retry_count, max_retries, runner_failure_count, timeout_seconds, condition, created_at, started_at, completed_at, error_message FROM tasks;
+DROP TABLE tasks;
+ALTER TABLE tasks_new RENAME TO tasks;
+CREATE INDEX idx_tasks_job_id ON tasks(job_id);
+CREATE INDEX idx_tasks_runner_id ON tasks(runner_id);
+CREATE INDEX idx_tasks_status ON tasks(status);
+CREATE INDEX idx_tasks_job_status ON tasks(job_id, status);
+CREATE INDEX idx_tasks_started_at ON tasks(started_at);

internal/database/migrations/000002_add_task_frame_end.up.sql

@@ -0,0 +1,2 @@
+-- Add frame_end to tasks for range-based render tasks (NULL = single frame, same as frame)
+ALTER TABLE tasks ADD COLUMN frame_end INTEGER;