feat(nav): add manual target controls

[xiaolefang-dm]

Objective

This is the first step toward measuring end-to-end navigation latency: from when the robot/app observes or selects a target, through planning and cmd_vel, to when the control layer begins executing it.

The manual target flow gives us a repeatable trigger for this measurement, so later instrumentation can compare timestamps across:

• Looper / visual input
• Looper bridge
• Planning
• cmd_vel generation
• Robot control execution

Summary

• Allow the operate tab to publish manual target poses to planning without requiring nav nodes to be running.
• Activate the nav control chain only when nav nodes are already running, and add a Stop Target control to clear manual targets.
• Disable manual target publishing during normal POI navigation so measurement/testing controls do not interfere with POI tasks.
• Prevent cleared manual targets from leaking NaN into planning websocket snapshots.
• Remove incomplete local Flutter fallback font config that caused 404s for missing Noto fallback fonts.

Testing

• python3 -m py_compile app/backend/node_manager.py app/backend/routers/nav.py tinynav/core/planning_node.py
• On nano test machine: flutter build web
• On nano test machine: restarted tinynav-dev and launched /tinynav/scripts/start_app.sh
• On nano test machine: verified /ws/planning returns valid JSON with grid/odom and no NaN target after clear
• On office robot dog: flutter build web
• On office robot dog: restarted tinynav-dev and launched /tinynav/scripts/start_app.sh
• On office robot dog: verified backend/frontend HTTP 200, manual target clear, and POI-navigation guard behavior

6d4b54a4c4960aba14e94257d365139d.mp4

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (240.7 files/s, 38234.5 lines/s)

Language	files	blank	comment	code
Python	13	672	313	3772
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	993	413	5265

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.18 s (236.4 files/s, 37541.0 lines/s)

Language	files	blank	comment	code
Python	13	672	313	3772
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	993	413	5265

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (241.2 files/s, 38303.4 lines/s)

Language	files	blank	comment	code
Python	13	672	313	3772
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	993	413	5265

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.20 s (210.6 files/s, 33468.4 lines/s)

Language	files	blank	comment	code
Python	13	672	313	3775
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	993	413	5268

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.18 s (234.8 files/s, 37309.9 lines/s)

Language	files	blank	comment	code
Python	13	672	313	3775
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	993	413	5268

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.18 s (241.4 files/s, 38657.6 lines/s)

Language	files	blank	comment	code
Python	14	694	313	3966
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	43	1015	413	5459

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.20 s (210.2 files/s, 33690.2 lines/s)

Language	files	blank	comment	code
Python	14	694	313	3971
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	43	1015	413	5464

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.18 s (234.0 files/s, 37526.0 lines/s)

Language	files	blank	comment	code
Python	14	694	313	3975
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	32	204
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	49
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	43	1015	413	5468