Add per-widget border option (style, thickness, palette color)

A Widget-level property (border_style/border_thickness/border_color_index),
not a per-type config field, since every widget type can have one -- drawn
once centrally in device.py's _render_widgets before compositing, using
an exact panel palette color so it never dithers. Styles: solid, dashed,
dotted, and a fancy double-line picture-frame-mat look. Configurable from
a shared "Border" card in every widget's gear-icon dialog.

server/app/routers/device.py

@@ -25,7 +25,7 @@ from .. import grid, mail, quiet_hours
 from ..auth import get_server_settings, require_device
 from ..db import frame_locked, get_db
 from ..firmware import firmware_path
-from ..image_pipeline import logical_render_size, render_panel, render_placeholder
+from ..image_pipeline import draw_widget_border, logical_render_size, render_panel, render_placeholder, resolve_border_color
 from ..models import BatteryLog, Frame, FrameButtonAction, Widget
 from ..widgets import WIDGET_TYPES
 from .common import (
@@ -81,11 +81,14 @@ def _setup_placeholder(frame: Frame, request: Request, manage: dict | None = Non
 def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wake: bool,
                     as_png: bool = False) -> bytes:
     """The widget-system compositor: renders every widget on this frame
-    into its own region (see app/grid.py for grid-cell -> pixel math) and
-    hands the results to image_pipeline.render_panel for the single
-    shared paste/enhance/overlay/quantize/pack pass. Replaces the old
-    per-mode RENDERERS dict -- a frame can now show several widgets at
-    once instead of exactly one mode owning the whole panel."""
+    into its own region (see app/grid.py for grid-cell -> pixel math),
+    draws that widget's own optional border directly onto its region
+    (models.Widget.border_style, a shared per-widget property no
+    widget_type module needs to know about) and hands the results to
+    image_pipeline.render_panel for the single shared paste/enhance/
+    overlay/quantize/pack pass. Replaces the old per-mode RENDERERS
+    dict -- a frame can now show several widgets at once instead of
+    exactly one mode owning the whole panel."""
     all_widgets = db.scalars(
         select(Widget).where(Widget.frame_id == frame.id).order_by(Widget.sort_order)
     ).all()
@@ -99,6 +102,10 @@ def _render_widgets(db: Session, frame: Frame, manage: dict | None, is_normal_wa
             frame.orientation, panel_w, panel_h, (widget.x, widget.y, widget.w, widget.h)
         )
         img = module.render(db, frame, widget, pw, ph, is_normal_wake=is_normal_wake)
+        draw_widget_border(
+            img, widget.border_style, widget.border_thickness,
+            resolve_border_color(widget.border_color_index, frame.palette_rgb),
+        )
         regions.append(((px, py, pw, ph), img))
     return render_panel(
         regions, orientation=frame.orientation, palette_rgb=frame.palette_rgb,